Springs are often used in orthodontic applications to supply a force for moving a patient's teeth. The force applied must be great enough for proper realignment of the tooth to occur. At the same time, however, when a patient's tooth is being moved, care must be taken not to apply too great a force to the tooth, otherwise resorption of the tooth root or some other harm to the tooth is likely to occur. Typically, posterior teeth require greater applied forces for movement than anterior teeth.
Springs of various shapes and sizes have been used throughout orthodontics, including finger springs, apron spring, coil springs, etc. In particular, coil springs made of stainless steel and nickel-titanium (Ni--Ti) alloys have been used in various orthodontic applications with a great deal of success even though these prior orthodontic coil springs have exhibited a number of disadvantages. Designing alternative coil springs with more optimum characteristics has proved very difficult.
Orthodontic coil springs made of stainless steel are relatively inexpensive and can be designed to provide sufficiently high applied forces to move a patient's teeth, even posterior teeth like molars. However, stainless steel coil springs are often unable to maintain a high enough applied force over a sufficient range of spring action (i.e., elastic change in spring length) without either some detrimental effect like tooth root resorption or the spring being too large to fit comfortably in the patient's mouth. In addition, because the force being applied by these springs typically diminishes very rapidly as the teeth start moving in the direction of force such springs have had to be replaced in order to obtain proper realignment of the teeth. Another disadvantage of stainless steel coil springs is that relatively low levels of deformation of the stainless steel material quickly results in permanent deformation of the spring. Stainless steels also contain elements, such as nickel, which have been known to cause adverse reactions in some patients.
Orthodontic coil springs have been successfully made of Ni--Ti alloys, but such springs are limited in their applications. Ni--Ti coil springs are typically able to maintain an applied load over broad ranges of spring action, but the forces applied by these springs are often too low to be useful in many orthodontic applications, in particular for distalizing posterior teeth such as molars. In addition, Ni--Ti alloys are relatively expensive and contain elements, such as nickel, which have been known to cause adverse reactions in some patients.
Therefore, there is a need in the orthodontic field for a more biocompatible coil spring design capable of exerting adequate applied forces over sufficient ranges of spring action to be useful in a greater number of orthodontic applications, including molar distalizing, without causing harm to the patient's teeth.
Reference has been made in the prior art, in particular U.S. Pat. No. 4,197,643, to the use of beta-phase titanium alloys for making orthodontic coil springs. Beta-phase titanium alloys are typically more expensive than stainless steel alloys and less expensive than Ni--Ti alloys. In addition, these alloys have well established biocompatibility. The '643 patent discloses a wide range of wire sizes, compositions and mechanical properties for use in making various types of orthodontic springs. The '643 patent, however, only suggests that coil springs made from such wire would be acceptable in orthodontic applications. Until the present invention, it was unknown which, if any, such beta-phase titanium alloy wires could be used successfully to make orthodontic coil springs. It was also previously unknown what, if any, coil spring designed with these wires would adequately perform in a broad enough range of orthodontic applications to supplant existing stainless steel and Ni--Ti coil springs.